As described in the above mentioned patent, copper is deposited at selected locations on a substrate, such as an epoxy-coated metal printed circuit "board," by coating the substrate with a sensitizer layer containing a reducible copper salt, such as copper formate, and exposing the layer to ultraviolet radiation through a mask so as to form a seed pattern of metallic copper at the exposed areas, which later acts as a deposition site for conventional electroless plating of copper on the seed pattern, for example in a process of making printed circuits.
The seed pattern comprises copper.sup.I or copper.sup.O converted from the copper.sup.II in the copper salt and it is desirable to be able to control the available copper.sup.II converted to the catalytic specie (copper.sup.I or copper.sup.O) during exposure to the ultraviolet radiation in order to aid deposition during the electroless plating.
In a preferred example, as described in the copending patent the sensitizer layer comprises a dried film including as principal constituents:
(a) copper formate, as the reductible copper salt;
(b) disodium 2,7 anthraquinone disulfonate, which acts as a primary reducing agent for the reaction; and
(c) sorbitol, which acts as a secondary reducing agent for the reaction and releases absorbed moisture.
As described in the copending patent, such compositions particularly those including 2,7 anthraquinone disulfonic acid or salts thereof are particularly effective in producing copper seed coating and are especially advantageous in providing a relatively long shelf life for the dried sensitizer film, such as seven days allowable time lapse between formation of the dried sensitizer film and the exposing step.